Phenotypic and genetic variation in the Dothistroma – Pinus pathosystem

Download

Date

Author

Metadata

Abstract

Trees and forests are under increasing threat from pathogens which cause
huge economic and ecological damage. The unprecedented global movement of
pathogens into new areas creates novel pathosystems, while the changing climate
affects the dynamics of endemic pathosystems. Co-evolution within endemic
pathosystems affects the genetic composition of hosts and pathogens. Spatial
heterogeneity in pathogen pressure leads to genetic variation in disease-related traits
among host populations. In contrast, novel hosts or populations are expected to be
highly susceptible to exotic pathogens as there has been no evolution of defence
responses. Host response to disease can therefore be an indicator of a novel or
endemic pathosystem. The long term resilience of forests to pathogens depends on
the adaptive capacity of both the host and pathogen species. Establishing the extent
of genetic and phenotypic variation within both the host and pathogen is therefore
fundamental in understanding past, current and future pathosystem dynamics.
The most significant current threat to Scots pine (Pinus sylvestris) is
Dothistroma needle blight (DNB) caused by the foliar pathogen Dothistroma
septosporum which is assumed to be exotic to Great Britain. This study aimed to
increase understanding of the genetic and phenotypic variation in this pathosystem.
Results from this study show that there are high levels of variation in the
Dothistroma – Pinus pathosystem. Genetic variation, elucidated using neutral genetic
markers, mating type specific markers and in vitro analysis of phenotypic variation in
D. septosporum collected from Scottish pinewoods, was found to be high: there was
high allelic diversity, particularly within plantation forests outside the native
pinewood range, and high phenotypic plasticity in response to different temperature
treatments. Both mating type idiomorphs were found in one forest which
demonstrates their potential for sexual as well as asexual reproduction. There is also
tentative evidence from this study that the pathogen is either introduced to Great
Britain or that endemic pathogen populations have been augmented with introduced
pathogens. Artificial and natural inoculations of native Scots pine provenances with D.
septosporum indicate that there is considerable variation in susceptibility to DNB
across the native range in Scotland and that variation in this trait is both highly
heritable and evolvable. Furthermore, provenance mean susceptibility to DNB is
negatively and significantly associated with water-related variables at site of origin,
a finding that is potentially indicative of a co-evolutionary history between host and
pathogen. Genetic differences among individuals which are ‘resistant’ or ‘susceptible’
to DNB were identified in Pinus radiata for which there has been extensive research in
this pathosystem, by comparing the transcriptome sequences of the two phenotypic
groups. Nearly half of the genetic differences identified among phenotypes were
found in genes with a putative defence function.
In conclusion, native Scots pine provenances contain the necessary heritable
genetic diversity to evolve a decrease in their susceptibility to D. septosporum through
natural selection in response to elevated prevalence of this pathogen. However,
implementation of key native pinewood management strategies, including
encouraging regeneration in particular, are necessary in order to facilitate the
adaptive evolution of native forests to increased levels of DNB. The effectiveness of
this response will depend on the rapidity of adaptation of the pathogen. Measures to
limit adaptation where possible, including the use of pathogen monitoring and
control in nurseries and the limitation of pathogen movement into native pinewoods,
should be continued.